Change of State and Latent Energy

During a phase change, the temperature of a substance does not increase or decrease. This is because the thermal energy during a phase change goes into breaking the intermolecular bonds between particles rather than increasing the kinetic energy of the particles. The latent heat is the energy released or absorbed during a change of state. ‘latent’ means hidden or unseen.

The latent heat is calculated using the formula: where: = the heat energy transferred in Joules (J) = mass (kg) = the latent heat (Jkg−1)

The value of L is dependent on the substance being considered and also the phase change. It takes a lot more energy to change a liquid to a gas than it does a solid to a liquid. This means the latent heat, L, for water to steam will be higher than for ice to water.

When considering a phase change from solid to liquid (melting), is described as the latent heat of fusion ( ). When considering a phase change from liquid to a gas (boiling), is described as the latent heat of vaporisation ( ).

Consider water:

• Latent heat of fusion ( ) = 3.34×105 Jkg−1
• Latent heat of vaporisation ( ) = 22.5×105 Jkg−1

Example 1:

How much energy is required to melt a 5kg block of ice?

using: where: = 5 kg = 3.34×105 Jkg−1   